What is an off-center-fed dipole?

Whereas a Dipole is a half-wavelength long radiator, fed in the Center of two equal length legs, the Off-Center-Fed Dipole (which I prefer to call a WINDOM) is a Dipole whose feed-point is at a point other than the center of the antenna.

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This will give you a general idea of how they work.

If you take an 80 meter 1/2 wave antenna and feed it in the middle, the impedance is around 72 ohms and an almost perfect match for 50 ohm coaxial cable. This same antenna is resonant on numerous other bands, such as 40, 20 and 10 meters. If you take this same antenna and use it on forty meters and keep the feed point in the center of the antenna, the feed point impedance will be around 3000 ohms. This is not a good match for coaxial cable. This antenna is a full wave antenna on forty meters but has to be fed at a different location if one wants to use 50 ohm coaxial cable. Move the feed point of this antenna so it is positioned one quarter wave length in from the end of the antenna. The feed point impedance of this antenna will again be about 72 ohms and is a good match for 50 ohm coaxial cable. If you use this antenna on 10 meters, again move the feed point of this antenna so it is one quarter wave in from the end. Again you will have a good match for 50 ohms coaxial cable and now have the makings of a long wire antenna with gain.

To make this same 80 meter dipole usable on numerous bands without moving the feed point for every band, you have to find a common point where numerous frequencies converge at a given impedance. By feeding this antenna one third of the way in from the one end, you will find this so called, “Sweet Spot”. The only thing left to resolve is the high impedance of this antenna at that point. The impedance’s of the numerous bands, on this antenna, will range from as low as 150 ohms up to around 300 ohms at this feed point. By placing a 4:1 Guanella Current Balun at this point you will have an almost perfect transformation of impedance so 50 ohm coaxial cable can be used. The end result would be an antenna that is usable on seven ham bands.

The overall length of this antenna is 135 feet. One leg of this antenna is 45 feet long and the other leg is 90 feet long. This will place the feed point of the antenna 45 feet in from one end.

This antenna was designed by K3CC! and it works!!!